This invention relates to a technique of producing electric pulses in accordance with the change of various physical quantities such as the position of a linearly or curvilinearly moving object or the angle of rotation of a rotary object. More particularly, the invention relates to a method and apparatus for producing a series of pulses in proportion to the change of a physical quantity, so that by counting the number of pulses it is possible to know the amount of change of the physical quantity.
There is known an instrument called an encoder for producing electric pulses in accordance with the change of the position or rotational angle of an object. There are two types of encoders, one of which is a rotary type while the other is a linear type. The former produces pulses in accordance with the change of rotational angle while the latter produces pulses in accordance with linear displacement, and both are graduated at regular intervals in the directions of rotation and linear displacement, respectively, so that electric pulses corresponding to the number of the graduation marks that have been passed upon rotation or displacement are produced.
Graduation is made in various ways, such as mechanical, optical or magnetic ways. In the prior art the graduation marks are at fixed, regular intervals. In order that the systems or instruments which employ encoders may operate stably, it is necessary that the graduation should be fixed. In rotary type encoders the number of graduation marks per revolution must be an integer to ensure continuity over a range of more than 360.degree..
The conventional encoders have the following disadvantages.
(1) In the conventional encoders the number of pulses produced per revolution or upon travel of a unit distance (which will be referred to as the pulse density or resolution hereinafter) is determined when the instruments are manufactured and cannot be changed later. Although quick operation is possible with an encoder having a high pulse density, it is difficult to make final positioning due to too quick motion, with a possibility of overshooting or a danger of collision of mechanical parts or elements. Although it is easy with an encoder having a low pulse density to locate a desired point, it takes a long time for the instrument to travel a long distance, or a desired travel cannot be completed by a single operation, with resulting reduction of the operability of the instrument. In practice, therefore, the pulse density of an encoder is set to a medium value in accordance with its use. However, since the pulse density is fixed, the operability of the instrument cannot be improved.
(2) To eliminate the above defect, there have been proposed several methods.
a) One of the methods is to use an electronic circuit to multiply the number of pulses by an integer so as to increase the apparent pulse density. However, the resulting pulse train lacks continuity, that is, even when the encoder is moved smoothly, the multiplied pulses are not produced evenly, but some of them are aggregated, so that a machine controlled by the pulses will move jerkily or vibration is likely to occur.
b) Another method is to divide the pulses by an integer to reduce the pulse density. This method also requires an electronic circuit for dividing pulses with resulting increase in the manufacturing cost of the encoder.
(3) Since the graduation of the known encoders is fixed, chattering is likely to occur in their operation to produce error pulses, which result in erroneous operation of the whole system in which the encoder is incorporated.